Mean-Time-Between-Failure (MTBF) and Duty Cycle for Fans
Fans, or blowers, are used to move air in a desired direction. Such fans can be used in a variety of applications, including but not limited to ventilating and cooling an area such as attic space, protecting components from overheating such as in a desktop, or larger, computer housing, evacuating humid or moist air such as in a bathroom after a shower, or evacuating hazardous gases or smoke from an area in a fire protection or other environmental control system.
Such fans are rated by a manufacturer specification of mean-time-between-failure (MTBF) designating the number of hours that the fan, on average, will run before it risks end of life types of failure. Thus, the MTBF of a particular type of fan is referred to as a predicted elapsed time between failures of the fan during operation, and it is calculated by the manufacturer as the arithmetic mean (average) time between failures for that type of fan.
MTBF is often expressed in connection with a duty cycle requirement for a fan. Duty cycle for a fan is defined as the ratio of the on-time fan operating duration to a total period (both on and off combined). Thus, a fan may be rated by its manufacturer with a particular MTBF assuming a duty cycle of no more than 75% on and 25% off for each hundred-hour period of fan life. If a particular system design requires fan usage in excess of the specified duty cycle for the fan, full useful life of the fan would not be achieved on average.
After a fan has run for the rated time according to its MTBF, failure may be anticipated and the fan may be replaced prior to failure during scheduled maintenance of the system. Thus, an understanding and application of MTBF, duty cycle and system maintenance are important since critical failure of a fan may lead to a variety of failures including but not limited to overheating damage, mold and mildew damage, and enhanced damage or injury from hazardous gases and smoke, depending upon the purpose for which the fan or fans have been deployed.
The replacement and repair of individual fans may in some cases be inconvenient, and especially so where the fans are not readily accessible, such as on a highly pitched rooftop or inside of a computer system that must be turned off in order to access the fans. One means of addressing the cost and inconvenience of fan failures has been to purchase higher-quality, typically more expensive, fans in order to increase MTBF. A downside to this approach is the higher up-front expense of installing and replacing such a system.
Fan Arrays
Another means of addressing the inconvenience and cost of replacing and repairing individual fans, as well as providing a higher-efficiency system for meeting system demands, has involved the development of redundant fan systems, or an array of fans, used in an effort to provide a longer-life system. Consideration of MTBF and duty cycle ideally applies to fan arrays, as well as single fans, and these concepts are an important part of scheduling repair, renewal and maintenance of fan arrays. However, prior art fan array systems have paid little or no heed to MTBF and duty cycle considerations in fan arrays.
Fully Redundant Fan Arrays
An array of fans may be fully redundant, partially redundant or not redundant at all. In a fully redundant system using switched identical fans, the overall MTBF of the system is increased by a multiple of the amount of redundancy employed. In the simplest of such systems using a plurality of identical fans, a first fan would simply run, or be available, until it fails, and then successive fans would be employed, and so on in a series-switched system, until the last fan in the system has been used up. In such a case, the overall MTBF of the array would have been the product of the number of fans and the MTBF of an individual fan: system MTBF=n×MTBF, where n=the number of fans. Where multiple types of fans with different MTBF characteristics have been employed in a single such system, the system MTBF would have been expressed as a sum of the MTBF for each fan in the array. Cost savings in a fully redundant fan array has been primarily associated with enhanced avoidance of failure associated damages and any economies of scale of volume purchasing, volume installations and volume repairs. More importantly, increase of lifespan of such fully redundant switched arrays beyond that normally expected by increasing the redundancy of fans in the array, has not occurred with and has not been a stated goal of such prior art fan arrays.
A suitable example of a fully redundant array system would comprise three identical fans, where one fan alone has provided sufficient capacity to meet demand, and two of the fans have been considered as backup fans. Absent volume purchasing and installation cost savings, such a system would have cost three times as much in terms of fan costs and installation as a single fan system, and the life of the array may have been expected to have lasted approximately three times as long as a single fan. Thus, though using such an array has been more convenient than a single fan from a maintenance and repair perspective, there has been no appreciable additional cost savings or enhancement of the life of the array beyond the sum of the MTBFs of each fan in the array.
The foregoing arrays are representative of fully redundant arrays, and the same basic MTBF and duty cycle considerations would hold true regardless of whether a single fan is used at a time or multiple fans are used at capacity at a time, assuming complete redundancy. Thus, for example, where two full duty cycle fans at capacity are needed to meet demands, and serial switching is employed, 3× redundancy would assume a system with six fans, and so on, and the same limitations on cost savings and enhanced system MTBF would apply.
Non-Redundant Fan Arrays
Non-redundant fan arrays involve a system where all fans in the array are used essentially at or near maximum duty cycle. While in such a case greater performance effectiveness and perhaps efficiencies have been accomplished by implementing an array, such a prior art array has not been employed so as to have enhanced the overall MTBF of the system beyond the rated MTBF of one of the fans of the system.
Partially Redundant Fan Arrays
Partially redundant arrays of fans have typically involved the use of control systems for determining the number of fans to be used in parallel, depending upon need, to meet requirements. Partially redundant arrays of fans have been developed primarily for HVAC systems where variable demands for cooling and ventilation have been required by an external device such as a thermostat, an emergency fire or hazardous air abatement and control system, a remote control device or a user-operated or automated switch. Accordingly, such systems have involved simultaneous use of multiple fans in a parallel array to meet varying system demands, for example for use during higher-temperature or higher-humidity periods.
Thus, for example, a partially redundant array of fans has comprised three fans that are all available in parallel to maintain thermostat readings in a cooling system at below a predetermined maximum. As the object of the cooling system, for example a building or a main frame computer system, has generated more heat than is able to be cooled with a single fan, additional fan power has been automatically added with control means responsive to data received from the thermostat. Further, such partially redundant fan arrays have incorporated application of remaining fan power upon encountering a failed fan in the system, similar to the serially-switched, completely redundant systems described above.
An example of such a system is found in U.S. Pat. No. 6,826,456 to Irving, et al., for System and Method For Controlling Server Chassis Cooling Fans, which teaches temperature sensing for controlling prior art fans that has included either an on-board, or an external, temperature sensor that has controlled the turning on and running of fans based upon increased temperature readings from the temperature sensor.
In conjunction with prior art fan array systems, whether they be fully redundant, partially redundant, or not redundant, there have been developed minimal signaling and reporting capability by failed fans. For example a system has been described in US Patent Application Serial No. 2006/0176186 to Larson et al., for Fan Monitoring for Failure Prediction, that has automatically notified an information technology professional when a fan of a fan array in a computer system is predicted to fail. This has allowed the IT professional to replace the fan or fan array, for example in a hot swappable fan system wherein the overall computer system has continued to operate while the fan, or fan array, has been removed and replaced.
Also, as demonstrated by U.S. Pat. No. 6,824,362 to Dodson III, for Fan Control System, other prior art fan array systems have employed an on-board tachometer that has been capable of actuating an alarm for under-speed fan conditions, thus signaling a potential critical fan failure or needed repair.
As may be appreciated from the foregoing description, partially redundant systems have presented a more complicated scenario for considering system MTBF and duty cycle, since each such partially redundant fan array system has presented widely diverging and unique capabilities to meet widely varying demands.
While there have been developed simple fan array controllers for determining the number of fans deployed responsive to an external device, such as a thermostat or a fire control system, and while there have been deployed fan array systems incorporating minimal fan data transfer and consideration by such a controller to accomplish simple management tasks, such as replacing a hot-swappable fan, a system for enhancing fan array life based upon static and dynamic fan data received from fans in the array has not been found in the prior art.
Thus, there has been lacking in the prior art a fan array system that has the intelligence for making determinations about which fans to use based upon demand, static fan data, such as rated MTBF, duty cycle and rated power consumption, rated temperature and rated speed, and dynamic fan data, such as hours in use, actual power consumption, revolutions per minute, and calculated remaining life, to enhance the MTBF of a fan system. Further, prior art systems have not allowed for inclusion of fans having differing MTBFs or other characteristics in a single array in a way that determines use based upon fan capability in order to enhance overall MTBF.